Gluconeogenesis

Question 1

What tissues rely on glucose solely as their main source of energy?

Answer 1

brain
nervous system
RBCs
testes
embryonic tissues

Question 2

What is gluconeogenesis?

Answer 2

generation of glucose from non-carbohydrate substrates e.g pyruvate, lactate, glycerol and glucogenic amino acids

Question 3

Where does gluconeogenesis normally occur?

Answer 3

the liver

Question 4

Gluconeogenesis is not the reverse of glycolysis as glycolysis has 3 irreversible steps. What happens instead?

Answer 4

there are 4 bypass reactions A,B,C and D

Question 5

What step of glycolysis are reactions A and B concerned with bypassing?

Answer 5

Step 10 - conversion of pyruvate or lactate to PEP

Question 6

What happens in reaction A?

Answer 6

conversion of pyruvate or lactate to oxaloacetate

Question 7

Lactate is covered to oxaloacetate in 2 steps. What are they?

Answer 7

lactate converted to pyruvate first by lactate dehydrogenase

pyruvate converted to oxaloacetate by pyruvate carboxlase

Question 8

How is reaction A different if pyruvate is used as the original substrate?

Answer 8

oxaloacetate is converted to malate to leave the mitochondrion and then is reconverted to oxaloacetate once outside the mitochondrion

Question 9

What happens in Reaction B of gluconeogenesis?

Answer 9

oxaloacetate is converted to PEP

Question 10

What difference is there in the progression of reaction B of gluconeogenesis when pyruvate is used as a substrate rather than lactate?

Answer 10

lactate - oxaloactetate is converted to PEP INSIDE the mitochondrion
pyruvate - oxaloacetate is converted to PEP outside the mitochondrion

Question 11

What step of glycolysis is Reaction C concerned with bypassing?

Answer 11

conversion of F-1,6BP to F-6-P

Question 12

What enzyme catalyses Reaction C?

Answer 12

fructose 1,6-bisphosphatase

Question 13

What reaction of glycolysis is Reaction D concerned with bypassing?

Answer 13

conversion of G-6-P to glucose

Question 14

What enzyme catalyses Reaction D?

Answer 14

G-6-Phosphatase

Question 15

How is free glucose formed?
Consider:
1. What is the usual end point for gluconeogenesis?
2. Where will free glucose be formed?

Answer 15

gluconeogenesis usually stops at G-6-P so that glucose can be ‘trapped’ in the cells
so free glucose formation will take place in the lumen of the ER

Question 16

Where can fructose enter glycolysis?

Answer 16

at F-6-P

Question 17

How is fructose metabolised?

Answer 17

by the fructose-1-phosphate pathway
catalysed by 2 enzymes: fructose-1-phosphate aldolase
triose kinase
1 or 2 ATP used per fructose molecule converted

Question 18

Where can galactose enter glycolysis?

Answer 18

at G-6-P

Question 19

How is galactose initially metabolised?

Answer 19

converted to G-1-P through sugar nucleotide derivative, UDP galactose

Question 20

What happens in the pentose phosphate pathway?

Answer 20

produces NADPH
produces pentoses (5C sugars) that are precursors of ATP, RNA and DNA

Question 21

The pentose-phosphate pathway has 2 phases. What are they?

Answer 21

• oxidative, irreversible bit that generates NADPH and converts G-6-P to a pentose phosphate
• an non-oxidative, reversible bit that in converts G-6-P and pentose phosphate to form lots of different 3,4,5,6, and 7 carbon sugars

Question 22

What enzyme is involved in the irreversible step of P-P-P?

Answer 22

G-6-P dehydrogenase

Question 23

What is the function of NADPH?

Answer 23

to link catabolic and anabolic pathways
catabolic= pentose phosphate pathway picks up H
anabolic= fatty acid synthesis uses H

Question 24

What is the difference between NADP+ and NAD+?

Answer 24

they are both electron carriers
NAD+ is used in metabolism
NADP+ is used in anabolism

Question 25

Why does drinking reduce gluconeogenesis?

Answer 25

the metabolism of ethanol requires NAD+ to act as an electron carrier
but the liver needs NAD+ for gluconeogenesis for conversion of lactate to pyruvate

Question 26

What are the adverse effects of reduced gluconeogenesis?

Answer 26

can lead to lacticacidaemia (increase in blood lactate concentration)
hypoglycaemia (decreased blood glucose concentrations)

Question 27

What enzyme deficiency causes Black Water Fever?

Answer 27

G-6-P dehydrogenase